function $(id) {
     return document.getElementById(id);
}
function sprawdz_dane() {
     var email = $("email").value;
     var zapytanie = "spr_email.php?email="+email;
     advAJAX.get({
          url : zapytanie,
          onSuccess : function(obj) {
          if(obj.responseText == "blad") {
               alert("E-mail zajety");
          }
          if(obj.responseText == "OK") {
               alert("E-mail wolny");
          }
                    }
     });
}

[PHP] pobierz, plaintext 
<?php
 
$sql=mysql_query("SELECT * FROM users WHERE email='".$_GET['email']."'");
					if(mysql_num_rows($sql)>0) {
						echo "blad";
					}
					else {
						echo "OK";
					}
 
?>
[PHP] pobierz, plaintext 

[HTML] pobierz, plaintext 
<tr>
					<td>E-mail: *</td>
					<td><input onblur="sprawdz_dane();" id="email" class="reg_w" type='text' name='email' value="<?php echo $_POST['email']; ?>" /></td>
				</tr>
[HTML] pobierz, plaintext